1. Field of the Invention
The present invention relates to a capacitive type differential pressure sensor having sensor cells with diaphragms made of a brittle material and constructed to permit batch processing.
2. Description of the Prior Art
Various pressure sensing devices have been advanced in the art which utilize deflecting diaphragms, the movement of which is detected to indicate pressure. Further, various pressure sensors, particularly differential pressure sensors, have utilized a fluid fill between the diaphragm and its support which deflect.
U.S. Pat. No. 4,177,496, for example discloses a capacitive pressure sensor comprising two discs made from a brittle insulator material, such as alumina, fused silica or glass, and which have spaced surfaces, facing each other, with conductive layers on such surfaces. A chamber is formed between the discs, and when the discs are subjected to external pressures the discs tend to move toward each other. The deflection of the discs is measured capacitively to provide an indication of the pressure acting on the discs. Portions of the discs are made to come into contact under overpressure to attempt to prevent damage to the sensor itself from excessive overpressures. A device similar to the one shown in U.S. Pat. No. 4,177,496 is also shown in U.S. Pat. No. 4,207,604.
A capacitive sensor shown in U.S. Pat. No. 4,168,518 is made of a material such as quartz and comprises two members that are joined together to form an interior chamber and which have portions that deflect toward each other in response to pressure acting on the exterior surfaces of the deflecting members.
U.S. Pat. No. 4,084,438 also shows a pressure sensing device which has spaced apart walls that are sealed at their peripheries to form an interior chamber, and capacitive sensing means on the inner surfaces of the walls, so that as the walls deflect under pressure and the spacing changes, an electrical signal is provided. The device shown in Pat. No. 4,084,438 further teaches the use of a material such as quartz for forming the sensor.
U.S. Pat. No. 4,332,000 shows capacitive pressure sensors formed from semiconductor material and discloses forming the sensor and the sensing circuity in the same process.
U.S. Pat. No. 4,390,925 also deals with a multiple cavity, pressure sensor formed on a silicon wafer.
U.S. Pat. No. 4,244,228 shows a pressure sensor which comprises slightly cupped shaped disc members that are joined at their peripheral edges and which will deflect under pressure. Under excessive pressures the plates will rest against each other. The discs are formed in a cup shape and made of a spring material in order to operate.
U.S. Pat. No. 4,301,492 shows a pressure sensor which utilizes diaphragms that are formed in a stacked arrangement, filled with fluid, and which will deflect relative to each other under pressure. This is sensed by capacitive sensors, and is compensated for errors due to temperature shifts. The chambers are formed by having the deflecting diaphragms spaced from each other by annular rims in a rest position. The sensor can be used for measuring differential pressures, but does require, in the form disclosed, recesses formed in the diaphragm discs to provide the necessary spacing.
When a fluid fill is used in a sensor, temperature changes affect the sensor due to the temperature coefficient of expansion of the fluid (usually a silicone oil) and the resulting changes in spacing of the diaphragm pressure sensing cells, which have sensing diaphragms on opposite sides of a center block, so that each of the diaphragms forms a separate chamber, with the chambers filled with fluid and fluidly connected and wherein differentials in pressure on the respective diaphragms will cause deflection of such diaphragms are shown in U.S. Pat. No. 4,398,194, and also in U.S. Pat. No. 4,169,389. Each of these patents teach that differences in temperature will cause different volumes of oil filling the space enclosed by the diaphragms. In U.S. Pat. No. 4,398,194 this difference in volume is compensated by the sensing circuitry, while in U.S. Pat. No. 4,169,389 the differences in volume are compensated for mechanically. In each of these patents a center, non-deflecting mounting block is utilized, as distinguished from the present device where chambers are formed from two plates, both of which will be deflected by external pressure variations.
U.S. Pat. No. 4,163,395 shows a sensor for sensing differential pressures that has flat diaphragms, the space between which is oil filled to set the diaphragm spacing. The diaphragms will "bottom out" under overload. An external sensor is used, apart from the deflecting diaphragm assembly, and the oil from the space between the diaphragms acts on the external sensor as pressure changes.
The use of brittle, semiconductor diaphragms on a rigid insulating substrate has also been advanced in absolute pressure sensors. A sensor of this type is shown in an article entitled "A Miniature Pressure Sensor For Automotive Applications", M. Behr and J. Giachino; I Mech E Conference Publications 1981-10 (1981), presented at The Third International Conference on Automotive Electronics at London in October 1981.
The prior art has examples of batch-fabricated absolute pressure sensors. The transition from making an absolute sensor of dissimilar materials, such as silicon and glass to a true differential batch fabricated sensor is a formidable task. The two sensor cells have to be attached to each other, stress isolated, oil filled and able to withstand high bidirectional overpressure of 1,000 to 10,000 psi and large line (static) pressures while accurately measuring (0.1 to 10 psi differential pressure. The present invention discloses structure for solving these problems.